Many automotive vehicles operating today and powered by internal combustion engines include an evaporative emissions control system. In such systems, vapors that form in the vehicle's fuel tank and associated portions of the fuel system are passed through a recovery canister containing carbon particles that remove or “scrub” hydrocarbons from the air before letting the air exit the fuel system. At certain times during vehicle operation, the vapor recovery canister is “purged” by forcing air though the carbon trap to desorb the hydrocarbons from the carbon, and that air/hydrocarbon mixture is then burned in the engine. Most current evaporative emission control systems operate with the fuel tank at or close to ambient atmospheric pressure, with the small amount of vapor pressure caused by fuel evaporation causing the flow of gasses through the canister. Such systems are referred to in this document as unpressurized.
It has been proposed to even further reduce evaporative emissions by isolating the fuel tank from the down-stream components of the evaporative emissions control system so that leakage of fuel vapors from the tank and related vapor recovery system lines and components is all but eliminated. When the tank is isolated in this manner, normal vaporization of the liquid fuel in the tank will generally cause the tank to become pressurized (above atmospheric pressure) to some degree. If the pressure in the fuel tank is above atmospheric pressure when the vehicle needs to be refueled, the tank pressure should be lowered to be at or near atmospheric pressure by opening an isolation valve so that the fuel vapors in the tank may flow to (and through) the recovery canister. If the positive pressure in tank is not relieved in this way before the refueling inlet is opened, the fuel vapors will escape through the inlet, thereby defeating the hoped-for reduction in evaporative emissions.